[FE] Make DiagnosticFactory.name not null

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticFactoriesInitializer.kt

@@ -1,16 +0,0 @@
-/*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
- * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
- */
-
-@file:Suppress("PackageDirectoryMismatch")
-
-package org.jetbrains.kotlin.diagnostics
-
-/**
- * This methods should me in `org.jetbrains.kotlin.diagnostics` package
- *   because of `DiagnosticFactory.setName` is package private
- */
-fun DiagnosticFactory<*>.initializeName(name: String) {
-    this.name = name
-}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticFactory.kt

@@ -16,7 +16,7 @@ import org.jetbrains.kotlin.fir.FirPsiSourceElement
 import org.jetbrains.kotlin.fir.FirSourceElement
 
 sealed class AbstractFirDiagnosticFactory<out E : FirSourceElement, D : FirDiagnostic<E>, P : PsiElement>(
-    override var name: String?,
+    override val name: String,
     override val severity: Severity,
     val positioningStrategy: SourceElementPositioningStrategy<P>,
 ) : DiagnosticFactory<D>(name, severity) {